Halogen mass flow rate detection system

ABSTRACT

A halogen mass flow rate detection system having an absorption cell through which a halogen entrained inert gas can be passed. An electromagnetic beam of preselected wavelength in the bound-continuum absorption region is passed through the cell while the halogen entrained inert gas is passed therethrough and also while only an inert gas is passed through the cell. By measuring the intensity of the beam passing through the absorption cell, molecular absorption can be utilized for determining the concentration and, therefore, the mass flow rate of the halogen passing through the absorption cell. By incorporating the halogen mass flow rate detection system in an iodine/oxygen chemical laser, for example, the mass flow rate of the iodine into the resonant cavity of the laser can be simply, accurately and reliably ascertained.

STATEMENT OF GOVERNMENT INTEREST

The invention described herein may be manufactured and used by or forthe Government for governmental purposes without the payment of anyroyalty thereon.

BACKGROUND OF THE INVENTION

This invention relates generally to flow rate detection systems, and,more particularly to a halogen mass flow rate detection system which canbe readily incorporated within a chemical laser.

Recent studies on chemical laser systems, and in particularoxygen/iodine chemical lasers have indicated a need for an accurate,diagnostic system for determining the amount of iodine (I₂) beinginjected into the laser or resonant cavity. Knowledge of the iodine massflow rate is essential in an understanding of the operation of such achemical laser. This is because the excited oxygen/iodine ratio isextremely critical in the operation of the laser, in that either toolittle or too much iodine will cause the efficiency of the laser to dropdrastically.

The present method of injection of iodine within the resonant cavityconsists of passing an inert gas such as argon over a container ofheated iodine (60° C.), thus entraining some unknown density of iodinetherein. This mixture is then passed into a slit nozzle for subsequentmixing with excited oxygen.

There are well-established techniques for measuring the amount ofexcited oxygen entering the laser but there are no techniques capable ofaccurately measuring the mass flow rate of iodine. A measurement of thetemperature of the iodine container does not give a reliable iodinedensity reading because of the nonequilibrium situation; i.e., asaturated vapor pressure above the iodine crystals is not achievedbecause of the rapid flow of the argon or inert gas carrier. Hence, onecannot even be certain that higher argon flow rates lead to higheriodine density being injected into the laser.

As pointed out hereinabove a determination of the mass flow rate ofiodine is essential to proper chemical laser operation. Morespecifically, too much iodine will cause the so-called "¹ Σ catastrophe"where the kinetics of the oxygen-iodine system are altered to such anextent that the laser will not operate. Too little iodine causes theextractable power to drop. In addition, knowledge of the iodine massflow rate greatly aids in the modeling of laser systems.

It is therefore readily apparent from the above recitation that aniodine mass flow rate detection system or, more generally, a halogenflow rate detection system is extremely desirable, and is of utmostimportance in, for example, the utilization of chemical lasers.

SUMMARY OF THE INVENTION

The instant invention overcomes the problems set forth hereinabove byproviding a simple and accurate diagnostic system for determining themass flow rate of a halogen such as iodine and, in addition, can bereadily coupled or incorporated into the iodine feedline of aniodine/oxygen chemical laser.

The halogen mass flow rate detection system of this invention utilizesan absorption cell through which an inert gas having entrained therein ahalogen is passed. In addition, passing through the absorption cell isan electromagnetic beam of radiation at a preselected wavelength in thebound-continuum absorption region. This invention utilizes molecularabsorption for determining the concentration of the halogen.

The technique of this invention utilizes absorption of the halogen on abound-continuum transition. By using the bound-continuum transition theabsorption coefficient is essentially constant. Thus, the measuredabsorbance varies linearly with the number density of the absorber. Thismeasurement is also extremely insensitive to temperature and added bathgas pressure which would ordinarily prevent severe complications in thebound-bound absorption transition. Consequently, the system of thisinvention is insensitive to carrier gas pressure and temperature whichare varied in actual chemical laser operation.

It is therefore an object of this invention to provide a detectionsystem which is capable of simply and accurately measuring the mass flowrate of a halogen.

It is a further object of this invention to provide a method for simplyand accurately measuring the mass flow rate of a halogen.

It is another object of this invention to provide a detection system formeasuring the mass flow rate of a halogen such as iodine and which canbe readily incorporated within a chemical laser system.

It is still another object of this invention to provide a halogen massflow rate detection system which is insensitive to temperature and gaspressure.

It is still another object of this invention to provide a halogen massflow rate detection system in which absorption measurements are made ona bound-continuum transistion.

It is still a further object of this invention to provide a halogen massflow rate detection system which is economical to produce and whichutilizes conventional, currently available components that lendthemselves to standard, mass producing manufacturing techniques.

For a better understanding of the present invention, together with otherand further objects thereof, reference is made to the followingdescription taken in conjunction with the accompanying drawing and itsscope will be pointed out in the appended claims.

DETAILED DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic representation, shown partly in cross section, ofthe halogen mass flow rate detection system of this invention;

FIG. 2 is a graph representative of the potential curves for iodinevisible transitions utilized in the technique of measuring mass flowrate of this invention; and

FIG. 3 is a schematic representation, shown partly in cross section, ofthe halogen mass flow rate detection system of this inventionincorporated within a chemical laser.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT AND METHOD

Reference is now made to FIG. 1 of the drawing which shows in detail thehalogen mass flow rate detection system 10 of this invention. Detectionsystem 10 includes therein an absorption cell 12 made up of an elongatedcylindrically-shaped tube 14 of predetermined length, L, and which hasoptically transparent windows 16 and 18 enclosing the ends thereof.Also, situated adjacent each end are entrance and exit ports 20 and 22,respectively, with the diameter of each of the ports being substantiallyequal to the internal diameter of elongated tube 14 of absorbtion cell12. Connected to entrance port 20 is an inlet line 24 while connected toexit port 22 is an exit line 26 which enables a halogen entrained inertgas to pass therethrough. In addition, operably connected to tube 14 isany suitable temperature sensor 28 in the form of, for example, aconventional thermocouple, and any suitable conventional pressure gauge30.

Reference is now made specifically to the inlet end of absorption cell12. Situated within inlet line 24, although its specific locationtherein is not essential, is any conventional halogen source which may,for example, in this invention be in the form of a container 32 forholding iodine crystals therein. At the furthest end of inlet line 24from absorption cell 12 is an inert gas source 34 in the form of, forexample, argon. Any suitable, conventional flow meter 36 isinterconnected within line 24 juxtaposed inert gas source 34 in order todetermine the mass flow rate of the inert gas (argon) as it passesthrough the halogen (iodine) on its way to the absorption cell 12.

In addition, a by-pass line 38 is interposed within inlet line 24 so asto provide a path for the inert gas to circumvent the halogen container32 when necessary during the procedure of halogen mass flow ratedetection as described in detail hereinbelow during the operation of thedetection system 10 of this invention. Conventional valves 40 and 42 arelocated within by-pass line 38 so as to enable the inert gas to passdirectly to the halogen container 32 or by-pass the halogen container32, if desired.

It should be noted, however, that although a by-pass line 38 is shown asa means of preventing the passage of the inert gas through the halogen,any other arrangement which would prevent the entrainment of the halogenby the inert gas under selected conditions would also be acceptable withthis invention. For example, it would be possible to freeze the halogencrystals in container 32 and thereby prevent their entrainment.

Optically aligned with one of the end windows 16 of absorption cell 12is any suitable, conventional laser 44 (such as an argon laser 44) whichis capable of passing a beam of electromagnetic radiation at apreselected wavelength through absorption cell 12. Receiving theelectromagnetic radiation passing through cell 12 and positionedjuxtaposed and in optical alignment with the other window 18 ofabsorption cell 12 is an intensity detector 46 in the form of, forexample, a conventional photomultiplier tube such as an RCA 4832 PMT.Electrically connected to photomultiplier tube 46 is any suitablecurrent flow recording or registering means 48 such as a conventionalmicroammeter. Microammeter 48 is capable of registering the current flowproduced by the change of intensity detected by photomultiplier tube 46.

It is essential for an understanding of the technique of mass flow ratedetection of this invention set forth in detail hereinbelow to firstunderstand the principal of optical absorption of radiation. Therefore,the following analysis is presented in abbreviated format.

Optical absorption of radiation from some ground state to an excitedstate by an ensemble of absorbing molecules is usually defined by someform of Beer's Law:

    I.sub.v (T)=I.sub.v (o)e.sup.-Nσ.sbsp.v.sup.L        (1)

where

I(T)≡intensity of probe after a single pass through the medium

I(o)≡initial intensity of probe beam

N≡number density of absorbtion molecules

σ_(v) ≡absorbtion cross section

L≡cell length

v≡optical frequency of probe beam.

This seemingly simple description of absorption is actuallyexperimentally very complicated because the actual absorption measureddepends upon the line shapes of both the absorber and probe.Consequently, the measured absorption is strongly influenced byexperimental conditions.

The situation is greatly simplified if the absorption is from a stableground state to a continuum of states. This can be illustrated with theaid of FIG. 2. FIG. 2 shows that the potential curves of the groundstate X¹ Σ and the excited B³ π(o) state dissociation energy is 20020cm⁻¹. This means that photons of wavelengths shorter than 4995 A will,when absorbed by I₂, dissociate the molecule into two iodine atoms. Forwavelengths longer than 4995 A, the absorption is between discretelevels and the absorption cross section depends upon both excited stateand ground state wave functions. This situation is extremely simple inthe bound-continuum absorption transition. Therefore, the instantinvention relies upon the bound-continuum absorption transitionillustrated by the arrow, A, in FIG. 2. The terminal level is in thecontinuum region since it lies above the dissociation energy. It can beshown that the absorption coefficient k for this case is given by thefollowing equation: ##EQU1## For the sake of brevity the constants ofthe above equation are not defined herein but should be recognized byone skilled in the art and can be found in Herzberg, G., Spectra ofDiatomic Molecules, Van Nostrand Reinhold Co., New York, NY, (1965) ifdesired. The important points to remember, however, are that theabsorption coefficient at a given frequency depends upon the numberdensity of ground state absorbers, N, and upon the vibrationalprobability distribution of the ground state set forth below: ##EQU2##

The term (dr/dv) is the gradient of the excited state potential curveevaluated at internuclear separation r, fixed by the absorptionfrequency v. The fact that the absorption coefficient depends only uponthe vibrational distribution of the ground state makes absorptionmeasurements fairly insensitive to the temperature of the absorbingmolecules and buffer gas pressure in the cell.

Reference is once again made to FIG. 1 of the drawing which illustratesthe absorption cell 12 of the halogen flow rate detection system 10.Absorption cell 12 is made of a known length, L. This is accomplished byplacing the entrance and exit ports 20 and 22 near the ends of theelongated tube 14 of cell 12. By making the diameter of entrance andexit ports 20 and 22 substantially the same as the diameter of the cell12 any gas dynamic effect of any suitable halogen such as iodine whichflows through the cell is minimized.

In the case of the mass flow rate detection system 10 of this inventionshown in FIG. 1 of the drawing the probe source utilized therewith is alow power argon-ion laser 44. An example of the operative power forlaser 44 may be only a milliwatt of power at a preselected wavelengthof, for example, 4880 A although substantially less power may actuallybe required. The length, L, of absorption cell 12 can be varied toaccount for different halogens or iodine concentrations, however, fornominal conditions a length, L, for tube 14 of only 5 through 10centimeters is ample. For lower concentrations, such an in kineticstudies, a larger tubular length can be used with this invention.

Since it is essential for the technique of this invention to establish anumber density, N, for the iodine in cell 12, it is necessary to measurethe absorbance of the 4880 A laser line, that is, at the bound-continuumabsorption region. The absorbance, A, is related to the number density,N, by the following formula:

    A=NσL

where N=number density of the molecular iodine;

σ=absorption cross section at the preselected wavelength of 4800A=1.6×10⁻¹⁸ cm² ; and

L=absorption cell length.

The above relationship has been found to be an accurate description ofthe absorption from 0-10 Torr of iodine and from 0-20 Torr of buffer gas(Ne). To determine the iodine mass flow rate out of exit line 26 asdepicted in FIG. 1 of the drawing it is also necessary to know theamount of inert gas such as argon in which the halogen such as iodine isentrained. The iodine is carried into the absorption cell 12 and fromcell 12 into the resonant cavity of a chemical laser (for example) bythe inert gas (argon) which passes over the halogen (iodine) incontainer 32.

In setting forth the steps of the method of this invention for rapidly,accurately and reliably determining the mass flow rate of a halogen suchas iodine entrained in an inert gas such as argon it is first necessaryto pass only the inert gas (argon) at a preselected mass flow rate,m_(IG), determined by flow meter 36 through the absorption cell 12 of apreselected tubular length, L. The temperature, T, in cell 12 isdetected by temperature sensor 28.

At the same time a determination is made of the pressure, P_(IG), of theinert gas by pressure gauge 30 within the absorption cell 12.Thereafter, a beam of electromagnetic radiation from laser 44 at apredetermined wavelength (i.e., at the bound-continuum absorption line)of, for example, 4880 A and of a predetermined power of, for example, amilliwatt is passed through absorption cell 12 while only the inert gas(argon) flows therethrough. This is accomplished by circumventing theiodine or halogen source in container 32 by means of by-pass line 38. Ameasurement of the intensity, I_(o), of the beam after being passedthrough cell 12 is detected by photomultiplier tube 46 and registered bymicroammeter 48.

Thereafter, the inert gas (argon) at the same preselected mass flowrate, m_(IG), is passed through the halogen (iodine) in container 32 inorder to entrain the halogen therein. The halogen entrained inert gasthereafter passes through absorption cell 12. The same electromagneticbeam of radiation from laser 44 is again passed through absorption cell12, however, at this time the halogen entrained inert gas is alsopassing through absorption cell 12. Measurement is now made of theintensity, I_(t), of the beam after being passed through the halogenentrained inert gas. The absorbance, A, of the halogen is now determinedby the following equation A=ln(I_(o) /I_(t)).

The number density, N_(H), of the halogen (iodine) is determined by theequation N_(H) =A/σL where σ is defined as absorption cross section ofthe inert gas at the predetermined wavelength 4880 A referred tohereinabove and which is published as 1.6×10⁻¹⁸ cm². A determination ofthe mass flow rate, m_(H), of the halogen which, for example, maypreferably be iodine, can be determined by the following equation m_(H)=m_(IG) (N_(H) /N_(IG)) wherein N_(IG) is the number density of theinert gas which, for example, may preferably be argon. The numberdensity of the inert gas, N_(IG), can be determined by the equation(P/760) (270° K./T) (2.68×10¹⁹) in which P is the pressure of the inertgas within cell 12.

Although the halogen mass flow rate detection system 10 of thisinvention is operable in and by itself it is extremely effective inmeasuring the mass flow rate of, for example, iodine within aniodine/oxygen chemical laser. Therefore, reference is now made to FIG. 3of the drawing which shows in detail the incorporation of halogen massflow rate detection system 10 of this invention within a chemical lasersystem 60. Chemical laser system 60 without the incorporation of halogenmass flow rate detection system 10 of this invention therein isconventional in its makeup and is of the type found in the publicationby W. E. McDermott et al entitled "An Electronic Transistion ChemicalLaser", Applied Physics Letters, 32 (8), Apr. 15, 1978.

Since the chemical laser system 60 is of conventional design only theelements necessary for use with halogen mass flow rate detection system10 will be described and have numerals associated therewith asillustrated in FIG. 3 of the drawing. Consequently, the remainingelements of chemical laser system 60 will not be found in FIG. 3 of thedrawing since the operation of chemical laser system 60 is conventionaland well within the purview of one having ordinary skill in the art.

More specifically, and still referring to FIG. 3 of the drawing, thelaser cavity or resonant cavity 62 is illustrated as an elongatedtubular element having reflective surfaces 61 and 63 at opposite endsthereof. An injection tube 64 is shown entering resonant cavity 62. Theinjection tube 64 allows for a passage therethrough of the iodine/argonmixture and excited oxygen as shown in the drawing. The excited oxygenenters through inlet 66 while the iodine in its crystalline form isfound in an iodine container 68. Any suitable inert gas such as argonemanating from argon source 70 is passed through a heater 80 beforepassing through the crystal iodine found in container 68. The argonentrains the iodine therein for subsequent passage through absorptioncell 12 before combining with the excited oxygen.

The halogen or iodine mass flow rate detection system 10 of thisinvention is interposed within the laser system 60 between the inletline 66 of the excited oxygen and the inlet line 82 of the iodine/argonmixture. The remaining elements of the halogen (iodine) mass flow ratedetection system 10 are identical to the elements shown in FIG. 1 of thedrawing with like references numerals being utilized in both FIGS. 1 and2 for identical elements.

The operation of the halogen (iodine) mass flow rate detection system 10utilized with chemical laser system 60 is identical to the operation ofhalogen mass flow rate detection system 10 described in detailhereinabove and therefore need not be repeated. Thus, the density of theiodine within the chemical laser 60 can be monitored as the argoncarrier flow is varied. By knowing the mass flow rate and pressure ofargon, the mass flow rate of the iodine entering resonant cavity 62 canbe easily and accurately determined by the formulae presentedhereinabove with respect to the operation of halogen mass flow ratedetector 10.

Although this invention has been described with reference to aparticular embodiment and methods, it will be understood to thoseskilled in the art that this invention is also capable of further andother embodiments within the spirit and scope of the appended claims.

We claim:
 1. A halogen mass flow rate detection system comprising anabsorption cell, means connected to said absorption cell for passingeither an inert gas or a halogen entrained inert gas thereinto, meansconnected to said absorption cell for passing either said inert gas orsaid halogen entrained inert gas out of said absorption cell, meansadjacent one end of said absorption cell for providing a beam ofelectromagnetic radiation at a predetermined wavelength and for passingsaid beam of radiation through said absorption cell, said wavelengthbeing in the bound-continuum region and said radiation being at apredetermined power, means adjacent the other end of said absorptioncell for receiving said beam of electromagnetic radiation passingthrough said absorption cell and for detecting the intensity thereof,and means operably connected to said absorption cell for measuring thepressure of said inert gas passing therethrough, whereby a relationshipbetween the detected intensity of said electromagnetic beam of radiationpassing through only said inert gas and the detected intensity of saidelectromagnetic beam of radiation passing through said halogen entrainedinert gas can be established, so as to then determine said halogen massflow rate.
 2. A halogen mass flow rate detection system as defined inclaim 1 further comprising means connected to said absorption cell formeasuring the temperature within said absorption cell.
 3. A halogen massflow rate detection system as defined in claim 2 wherein said absorptioncell comprises an elongated tube having an internal diameter of apreselected size and said inert gas or halogen entrained inert gaspassing means comprises a hollow member having an internal diameterwhich is substantially the same as said preselected size.
 4. A halogenmass flow rate detection system as defined in claim 3 and furthercomprising an inert gas source connected to one end of said inert gas orhalogen entrained inert gas passing means, and the other end thereofbeing connected to said absorption cell.
 5. A halogen mass flow ratedetection system as defined in claim 4 and further comprising a halogensource connected to and intermediate the ends of said inert gas orhalogen entrained inert gas passing means.
 6. A halogen mass flow ratedetection system as defined in claim 5 wherein said inert gas or halogenentrained inert gas passing means further comprises means forselectively circumventing said halogen source.
 7. A halogen mass flowrate detection system as defined in claim 6 wherein said means forselectively circumventing said halogen source comprises a bypass linehaving a pair of valves therein.
 8. A halogen mass flow rate detectionsystem as defined in claim 7, and further comprising means connected tosaid intensity detecting means for registering said intensity of saidelectromagnetic beam passing through said absorption cell.
 9. A halogenmass flow rate detection system as defined in claim 8 wherein saidelectromagnetic beam providing means comprises a laser.
 10. A method ofrapidly, accurately and reliably determining the mass flow rate of ahalogen which is entrained in inert gas comprising the steps of:(a)passing an inert gas at a preselected mass flow rate, m_(IG), through anabsorption cell of preselected length, L, and at a preselectedtemperature, T; (b) determining the pressure, P_(IG) of said inert gaswithin said absorption cell; (c) passing a beam of electromagneticradiation of predetermined wavelength in the bound-continuum region andat a predetermined power through said absorption cell while only saidinert gas flows therethrough; (d) detecting the intensity, I_(o), ofsaid beam after being passed through only said inert gas in saidabsorption cell; (e) passing said inert gas at said preselected massflow rate through a halogen in order to entrain said halogen therein;(f) passing said halogen entrained inert gas through said absorptioncell; (g) passing said beam of electromagnetic radiation through saidabsorption cell while said halogen entrained inert gas flowstherethrough; (h) detecting the intensity, I_(T), of said beam afterbeing passed through said halogen entrained inert gas; (i) determiningthe absorbance, A, of said halogen by the equation A=ln(I_(o) /I_(t));(j) determining the number density, N_(H), of said halogen by theequation N_(H) =A/σL where σ is defined as the absorption cross sectionof said inert gas at said predetermined wavelength; and (k) determiningsaid mass flow rate, m_(H), of said halogen by the following equation:m_(H) =m_(IG) (N_(H) /N_(IG)) wherein N_(IG) =number density of saidinert gas=(P_(IG) /760) (270° K./T) (2.68×10¹⁹).